1. Field of the Invention
The present invention relates to methods and compositions for reducing the permeabilities of subterranean formations or zones penetrated by well bores.
2. Description of the Prior Art
When wells penetrating hydrocarbon producing formations or zones are produced, water often accompanies the hydrocarbons, particularly as the wells mature in time. The water can be the result of a water producing zone communicated with the hydrocarbon producing formations or zones by fractures, high permeability streaks and the like, or the water can be caused by a variety of other occurrences which are well known to those skilled in the art such as water coning, water cresting, bottom water, channeling at the well bore, etc. It becomes an economic necessity to reduce the water production such that recovery of remains cost effective.
In enhanced recovery techniques such as water flooding, an aqueous flood or displacement fluid is injected under pressure into oil containing subterranean formations or zones by way of one or more injection wells. The flow of the aqueous fluid through the formations or zones displaces hydrocarbons contained therein and drives them to one or more producing wells. However, the aqueous displacement fluid often flows through the most permeable formations or zones whereby less permeable formations or zones containing hydrocarbons are bypassed. This uneven flow of the aqueous displacement fluid through the formations or zones reduces the overall yield of hydrocarbons therefrom. Heretofore, enhanced recovery problems in subterranean hydrocarbon containing formations or zones caused by permeability variations therein have been corrected by reducing the permeabilities of the subterranean flow paths having high permeabilities and low hydrocarbon content. As a result, the subsequently injected aqueous displacement fluid is forced through flow paths having low permeability and high hydrocarbon content. The techniques utilized to accomplish this high flow path permeability reduction, referred to in the art as “conformance control techniques”, have included injecting aqueous solutions of polymers and gelling agents into the high permeability flow paths whereby the polymers are gelled and cross-linked therein. For example, water soluble polymers including copolymers of acrylamide and acrylic acid cross-linked with transition metal ions have been utilized heretofore. However, when such fluids are utilized in subterranean formations or zones having temperatures above about 200° F., the cross-linkers containing metals or the polymers become unstable. This has resulted in uncontrolled cross-linking rates which cause cross-linker precipitation, polymer degradation, etc. While other non-metal cross-linking agents have been developed and used, they have also suffered from very rapid cross-linking at temperatures above about 200° F. and instability problems brought about thereby. As a result of the short cross-linking times of the conformance control polymers and gelling agents utilized heretofore in high temperature formations or zones, it has been common practice to cool the formations or zones such as by injecting water into the formations or zones prior to introducing the conformance control polymer and cross-linking gelling agent therein. While cooling the subterranean formations or zones has allowed successful placement by injection conformance control, the cool down process is time consuming and expensive to carry out.
Thus, there are needs for improved methods and compositions for reducing the permeabilities of subterranean formations or zones without the requirement of cooling the subterranean formations or zones prior to introducing conformance control displacement fluids therein.